The invention relates to the field of cementing plugs downhole in wellbores. More particularly, the invention relates to an improved cementing plug system for identifying the location of one or more cementing plugs downhole in a wellbore.
Cement plugs and wipers are placed in a wellbore to facilitate and control the positioning and cement, fluids and other materials downhole in the wellbore. In cementing operations, a bottom plug is positioned in wellbore casing and liquid cement is pumped into the casing to move the plug downhole. The bottom plug displaces mud used in drilling operations and wipes the interior surface of the casing string. When the bottom plug reaches floating equipment such as a float collar, the fluid pressure differential ruptures the bottom plug to permit movement of the cement through the bottom plug and into the annulus between the casing and the wellbore rock.
As the cement is pumped into the casing, a top cement plug is often released into the casing to follow the cement and to wipe cement from the casing interior wall. When the top plug reaches the float collar further progress is impeded so that a resulting pressure increase notifies the operator that the top cement plug has reached the selected destination.
Many variations of cement plugs have been developed. For example, U.S. Pat. No. 6,196,311 to Treece (2001) described an improved cementing plug which can be used as a lower or top plug. U.S. Pat. No. 6,237,686 to Ryll et al. disclosed an elastomeric cement plug. U.S. Pat. No. 6,244,350 to Gudmedstad et al. (2001) disclosed a cement plug having hollow upper and lower sections and ports for selectively placing cement in a wellbore. U.S. Pat. No. 6,263,968 to Freeman et al. disclosed a combination of top and bottom cement plugs for placing cement in a wellbore.
Nonrotating cement plugs are used when polycrystalline diamond compact drill bits remove the plug and excess cement from the casing interior. U.S. Pat. No. 4,858,687 to Watson et al. disclosed a nonrotating cement plug, and U.S. Pat. No. 5,165,474 to Buisine et al. (1992) disclosed a cementing plug having deformable fins for resisting rotation of the plug.
To cement a casing in a wellbore, volumetric calculations consider the borehole dimentions and depth of the float valve. If a displacement plug does not reach the seat when the calculated total displacement volume has been pumped, pumping operations are typically stopped so that overdisplacement of the cement does not occur. Overdisplacement of the cement moves all of the cement outside of the casing. Underdisplacement of the cement leaves cement within the casing, requiring expensive and time consuming drilling operations to remove the misplaced cement. U.S. Pat. No. 5,095,988 to Bode (1992) disclosed a ball injection apparatus for launching cement plugs into well casing. Upper and lower plug assemblies sequentially launched balls, bombs or darts into the cement to control cement placement. To verify the location of the balls or other devices in the cement, a magnetic sensor was placed downhole to detect small magnets implanted in the balls and to trip a light indicating passage of the ball or other device.
Various techniques have been proposed to control the amount of cement pumped into a wellbore. For example, U.S. Pat. No. 6,170,574 to Jones (2001) disclosed a pump bailer for transporting cement downhole into the wellbore and for releasing the cement at a selected position within the wellbore.
Other techniques use wireline logging tools to locate the position of a cement column within a casing. For example, U.S. Pat. No. 6,189,383 to Tello et al. (2001) described a wireline logging tool for measuring the density of fluids downhole in a casing.
Various techniques lock a tool into a selected position downhole in a casing. Mechanical locking techniques are well known as a technique to positioning a tool, and one form of mechanical location device is identified in U.S. Pat. No. 6,199,632 to Shy (2001). Another form of mechanical locking device was disclosed in U.S. Pat. No. 4,491,178 to Terrell et al. (1985), which disclosed a wireline controlled setting tool having an explosive mechanism for activating operation of the tool downhole in the wellbore.
Cementing operations are subject to many variables. Variances in internal pipe diameter can significantly affect the column height of cement within the pipe. Recognized displacement factors do not account for the volume within each tool joint, leading to additional discrepancies in cement placement. In pipe including collars, drift in the internal diameter and gaps between the pipe and couplings create additional variables in the pipe string interior volume. Obstructions in the pipe string interior can cause a cement plug from landing short of the correlative landing collar. Lost circulation, washouts, and obstructions can interfere with cement placement, thereby jeopardizing the accuracy of identifying desired cement placement within the wellbore.
The invention provides a system for identifying the location of a body in a wellbore extending downwardly from the surface through subsurface geologic formations. The system comprises a body moveable in the wellbore, a transmitter for sending a signal through the wellbore which identifies the location of the body within the wellbore, a receiver for collecting the signal, and a processor engaged with said receiver for processing said signal and for identifying the location of the body within the wellbore.
In another embodiment, the invention provides a system for identifying the location of a cement plug in a wellbore extending downwardly from the surface through subsurface geologic formations, and comprises a body moveable in the wellbore, a transmitter attached to the body for sending a signal through the wellbore which identifies the location of the body within the wellbore, a receiver for collecting the signal, and a processor engaged with the receiver for processing the signal and for identifying the location of the body within the wellbore.